Objective: The objectives of these etiologic studies are to assess the role of diet (including fat, fiber, fruits and vegetables, selenium, flavonoids, and several vitamins and carotenoids) in breast cancer, colorectal cancer, and lung cancer. Background: The important relationship of diet and nutrition in the development of cancer has become well known through various research efforts. Laboratory studies have shown cancer inhibitory function for various natural and synthetic nutrients in various models, which have been corroborated by human epidemiologic studies of nutrient intake, tissue levels, and cancer incidence. Methods: The project encompasses two studies. The first is a population- based, incident breast cancer case-control study in Helsinki, Finland, that utilized a modified dietary history questionnaire, anthropometric measurements, and other reproductive questionnaire data. Women diagnosed with benign breast disease were also studied. The second study is based on a large population-based cohort (n=10,000) through the Social Insurance Institute, with prediagnostic baseline dietary histories and other risk factor data. Progress: Results concerning dietary fat intake have been reported from the cohort study and showed an inverse association between energy intake (and positive association for dietary fat) with breast cancer risk. Mono- and poly-unsaturated fats contributed to the association more than saturated fats. In the same study, inverse gradients were observed between lung cancer and intake of fruits, carotenoids and vitamin E and C in nonsmoking men. No associations were seen for dietary retinol or selenium, or among smokers. From the case-control study, reproductive risk factors have been analyzed and reported which support established risk associations, while dietary and anthropometric analyses are pending. The pilot study of breast adipose tissue fatty acid and micronutrient levels demonstrated no differences in tissue levels across breast quadrants or by proximity to breast tumors.